This invention relates to a capless multiaxial screw and spinal fixation assembly and method, particularly useful for fixing and/or aligning vertebrae of the spine. The invention permits multiple angular orientations of an elongated member or rod with respect to a screw that is screwed into a vertebra.
Various methods of spinal immobilization have been known and used in the past. The preferred treatment for spinal stabilization is immobilization of the joint by surgical fusion or anthrodesis. This method has been known since development in 1911 by Hibbs and Albe. However, in many cases, in particular cases involving fusion across the lumbosacral articulation and where there are many levels involved, pseudorarthrosis is a problem. It was discovered that immediate immobilization was necessary in order to allow a bony union to form. Post operative external immobilization, such as the use of splints and casts, was a favored method of treatment, however, as surgical techniques have become more sophisticated, various methods of internal and external fixation have been developed.
Internal fixation refers to therapeutic methods of stabilization which are wholly internal to the patient and include commonly known devices such as bone plates and pins. External fixation, in contrast, involves at least some portion of stabilization device which is external to the patient's body. Internal fixation is now the favored method of immobilization because the patient is allowed greater freedom with the elimination of the external portion of the device and the possibility of infection, such as a pin tract infection is reduced.
There have been numerous systems and methods developed in the past for correcting and stabilizing and aligning the spine for facilitating, for example, fusion at various levels or areas of the spine, such as those devices are shown in U.S. Pat. Nos. 4,085,744; 4,269,178; 4,805,602; 5,466,237; 5,474,555; 5,891,145; and 6,869,433 B2. Bone screws with a polyaxial head are commonly used in spine surgery today. They are used chiefly in the lumbar spine and screwed into bone (pedicle) posteriorly. The head of the screw is attached to the shaft of the screw by means of a ball and socket. The top of the screw is machined into a ball, and the head contains a socket into which the ball fits. The screw head further contains a receiver for receiving a separate rod. The rod is fastened to the screw head receiver via a threaded cap. The rod is then fastened to screws placed in adjacent vertebrae thus providing stabilization. The polyaxial head allows the rod to be placed in a variety of angles with respect to the screw allowing conformance to local anatomy.
When the threaded cap is tightened upon the rod, a frictional pressure is transmitted from the threaded cap to the rod thence to the top of the ball, thus locking the ball-in-socket and preventing motion after tightening has occurred. This concept is demonstrated in U.S. Pat. Nos. 5,466,237 and 5,474,555, which illustrate this type of screw.
U.S. Pat. No. 5,466,237 to Bird et al. discloses a bone screw having a spherical projection on the top of the bone screw. An externally threaded receiver member supports the bone screw and spinal rod on top of the spherical projection. An outer nut is tightened onto the receiver member to press the spinal rod against the spherical projection to accommodate various angular orientations of the bone screw relative to the rod.
In another approach shown in U.S. Pat. No. 4,946,458 to Harms, a spherical headed bone screw supported within separate halves of a receiving member. The bottom of the halves are held together by a retaining ring. The top of the receiver halves are compressed about the bone screw by nuts threaded onto a threaded spinal rod.
In still another approach taken by Harms et al. in U.S. Pat. No. 5,207,678, a receiver member is flexibly connected about a partially spherical head of a bone screw. Conical nuts on opposite sides of the receiver member threaded onto a threaded rod passing through the receiver. As the conical nuts are threaded toward each other, the receiver member flexibly compresses around the head of the bone screw to clamp the bone screw in its variable angular position. One detriment of the systems in the two Harms et al. patents is that the spinal rod must be threaded in order to accept the compression nuts.
U.S. Pat. No. 6,869,433 discloses the use of a pedicle screw assembly that comprises a screw having a head with a convex portion and a receiver that receives the head. The receiver also receives an elongated member, such as a spinal fixation rod. The receiver has a concave portion which has a radius of curvature which is less than the radius of curvature of the convex portion of the head whereby to create an interference fit between the convex portion of the head and the concave portion of the receiver. The device also includes an internal nut and external nut that compresses the rod against a pressure disc which in turn compresses the head convex portion of the screw into the receiver concave portion and locks the angular position of the receiver with respect to the screw.
One of the problems with the prior art devices is the number of parts and components, especially those components that utilize a threaded cap screw to secure the rod to the anchoring screw, whether internal or external, to fix the rod relative to the screw. Problems with the threaded fastener, that is, threaded cap or set screw, are numerous and include risk of cap loosening, loss of cap intra-operatively, cross threading, thread failure, failure of the cap in driving instrument and limitations upon torque application.
What is needed, therefore, is a system and method that provide a lock or connection between the rod and screw without the use of external nuts, screws, caps or threads of the type shown in the prior art.
These and other objects and advantages of the invention will be apparent from the following description, the accompanying drawing and the appended claims.